Certain protein-containing food products, such as acidified dairy products like drinking yoghurt and stirred yoghurt, require a stabiliser to stabilise the protein system against aggregation, sedimentation and separation. The major protein present in cows' milk is casein, which constitutes about 80% of the total protein content. The remaining protein in cows' milk is termed “whey protein” and consists predominantly of beta-lactoglobulin and alpha-lactalbumin. Cows' milk is comprised of water and milk solids. The milk solids include fat and milk solid non-fat (MSNF) which is made up of protein together with lactose and various minerals.
Pectin has traditionally been used as a stabiliser in protein-containing food products such as acidified protein beverages (typically high ester pectin) and stirred yoghurt (typically low ester pectin). Pectin is a structural polysaccharide found in green land plants, for example, fruit and vegetables and may be extracted from citrus fruit peel. At a molecular level, pectin consists of a linear chain of galacturonic acid units linked through α-1,4 glycosidic bonds (the ‘smooth region’). This regular structure is interrupted by rhamnopyranosyl residues with side chains of neutral sugars (the ‘hairy region’). Pectin molecules have a molecular weight of up to about 200,000 and a degree of polymerisation of up to about 800 units. A proportion of the carboxylic acid groups of the galacturonic acid units are methyl esterified. The properties of pectin depend on the degree of esterification, which is less than 50% for low-ester (LE) pectin and more than 50% for high-ester (HE) pectin.
Pectin is known to have the ability either to prevent aggregation of casein micelles or to be the cause of it, depending on the pH of the system. The micellar casein-pectin system switches from hydrocolloid non-adsorption and depletion flocculation at neutral pH 6.7 to hydrocolloid adsorption and polymeric stabilisation at pH 4 [2, 4]. Therefore, although pectin is an effective stabiliser at acidic pH, at neutral pH conditons it is incompatible with the milk proteins and separates the milk into two phases.
Depletion flocculation of casein micelles involves exclusion of the polymer pectin chains from the space between the colloidal casein micelles, which induces an attractive interaction between the casein micelles. If the depletion attraction is strong enough, segregative phase separation occurs resulting in two immiscible aqueous phases, where the upper phase is rich in pectin and poor in casein micelles, while the lower phase is, on the contrary, mainly loaded with casein micelles [1, 2, 4]. At a low pectin concentration, the phase volume occupied by the pectin molecules is low. At increasing pectin concentrations, the occupied volume and the osmotic pressure of the pectin solution increase, which induces a stronger flocculation of the casein micelles. Finally, at a certain pectin concentration, the phase separation takes place. About 0.20% HE pectin is needed to induce phase separation in skimmed milk at pH 6.7 [2].
Pectin is a non-adsorbing polymer when it is in solution with skimmed milk at pH 6.7, but when lowering the pH to 5.3, the pectin molecule adsorbs onto the casein micelle. If the pectin concentration is low and insufficient for full coverage of the casein micelles, bridging flocculation occurs. When increasing the pectin concentration further, the casein micelles become fully coated and the system re-stabilises. Thereby, the attraction between the casein particles is lowered and stable conditions are obtained [2]. Although the adsorption of pectin onto the casein micelles is possible at pH conditions above the isoelectric point of caseins (pI˜4.6), the pH of efficient stabilisation is generally restricted to about pH 3.5 to 4.4 where the pectin and casein carry sufficient opposite net charges for effective adsorption [4].
This mechanism is used to stabilise acidic protein beverages against protein aggregation. Efficient polymeric stabilisation is achieved by the combination of high molecular weight, high surface coverage and a blockwise distribution of galacturonic acid groups. Therefore in theory, the best polymeric stabiliser would be a copolymer with a strongly adsorbing terminal with low solvent affinity and a voluminous dangling end with high solvent affinity to increase repulsion upon forced polymer overlap [4]. For stabilisation of acidic protein beverages, HE pectin has generally been considered to be the hydrocolloid of choice. Although HE pectin has a lower charge density than low-ester (LE) pectin and thereby a weaker electrostatic interaction with casein micelles, it generally serves as a more effective stabiliser of casein dispersions. It is believed that a smaller region of the HE pectin molecule interacts with the casein particle, allowing a more substantial part of the pectin dangling chain to be freed from solvent interaction thus preventing protein aggregation through steric hindrance [7].
The difference in the stabilisation characteristics of HE pectin at different pH values determines the applications in which HE pectin may be used as a stabiliser and the stage in the production process when the HE pectin may be added.
The acidification of protein beverages can be achieved by the addition of an acid (for example an acidic fruit juice). Acidification can also be achieved via fermentation. However, for acidified protein beverages containing HE pectin, these two processes are technically distinct from each other:
For directly acidified protein beverages like milk juice drinks, addition of juice and/or acid directly to milk results in the formation of acid casein particles of uncontrollable size. These particles are typically too big to be kept in suspension resulting in a non-stable acidic protein beverage with a sandy mouth-feel upon heat treatment. In the production of directly acidified protein beverages, the destabilising effect of high molecular weight HE pectin at neutral pH is used to advantage. The HE pectin is typically added to the milk before acidification and, under the neutral pH conditons, induces separation of the milk into two phases. The osmotic effect of pectin concentrates the intact casein micelles in a lower, protein rich phase and leaves the pectin-rich whey phase virtually free of micelles. The casein phase has the properties of a liquid and can be dispersed into the whey phase in the form of droplets by stirring. The more shear applied to the system, the smaller the drops become and the more like an oil-in-water emulsion the system becomes. The subsequent rapid pH drop through direct acidification freezes the casein droplets in their native form at the size they had in the neutral milk and thereby creates acid casein particles of controlled size [5]. During the acidification process natural stabilisation of casein is destroyed and the presence of HE pectin that forms the above-mentioned protective coat around the casein micelles prevents aggregation and precipitation [13].
Thus, for directly acidified protein beverages, HE pectin is added at neutral pH and induces phase separation. Strong mechanical stirring is then used to keep the precipitated casein proteins in suspension. The system is rapidly acidified freezing the casein proteins in suspension. The casein proteins are stabilised by the high-ester pectin molecules under the acidic conditions and are thereby prevented from sedimentation in the final application.
For fermented milk products, HE pectin cannot be used in the same way. Production of fermented milk products typically involves the steps of pasteurisation of the milk base, followed by inoculation with bacteria and finally fermentation. During fermentation by bacteria, the pH of the milk is reduced gradually and slowly in contrast to the rapid pH drop in the above application. Thereby, a disintegration of the casein micelles takes place that thickens or gels the milk into yoghurt [5, 13].
Addition of traditional, high molecular weight HE pectin to the milk before fermentation would induce phase separation as described above, when applied in concentrations required for efficient protein stabilisation of the final fermented drink. Phase separation in this application would be undesirable because the characteristic yoghurt structure and its subsequent texture impact would be lost. Furthermore the precipitated casein micelles cannot be kept in suspension by stirring in this application. Mechanical stress and incorporation of oxygen is normally avoided during fermentation of milk to give the live bacteria the best fermentation conditions. Therefore, strong mechanical stirring to keep the separated casein micelles in suspension cannot be applied. Moreover, the pH drops too slowly to freeze the casein structures. In summary, high molecular weight HE pectin is not typically effective if added to milk before fermentation and is instead added after fermentation to protect the acidified proteins against aggregation [14].
For fermented milk products like stirred yoghurt the typical choice of pectin stabiliser is LE pectin that provides both stability and texture to the fermented protein system. In neutral milk the phase separation boundary is obtained at even lower pectin concentrations when LE pectin is applied [16]. In practice about 0.15% LE pectin can be added to neutral milk without phase separation. However, this low dosage is often not sufficient to obtain a required high viscosity and creaminess in the resulting fermented milk product like stirred yoghurt. Moreover, the request for improved viscosity and creaminess becomes even more relevant when the solid milk ingredients like proteins and fat are reduced in the formulation for the purposes of cost reduction or calorie reduction.
For fermented dairy products which contain live culture the final product is not typically pasteurised or sterilised. It is therefore of utmost importance to pasteurise the milk prior to fermentation, to avoid contamination during fermentation and contamination of the final product. When pectin is applied to fermented milk drinks containing live culture, it must be sterilised as well to avoid contamination of the product. As discussed above, known commercial pectin products cannot be added to the milk prior to pasteurisation, inoculation and fermentation and therefore the pectin needs to be sterilised separately. This typically involves the heat sterilisation of aqueous pectin solutions that require additional processing and equipment to both dissolve and heat the pectin. The pectin is typically in the form of a pectin syrup which is sterilised by heating and subsequently added to the already fermented milk base. The additional pectin sterilisation process requires additional tank capacity and heat equipment and increases the energy costs. The alternative and much simpler method of adding pectin directly to the fermented milk in the form of a dry mix with sugar is not applicable due to the contamination risk.
For manufacturers of fermented milk products it would be easier and cheaper (e.g. in terms of process equipment and energy requirement) to operate with a stabiliser which can be added to the milk prior to fermentation i.e. before the slow acidification. Before fermentation, it is common to pasteurise the milk in order to avoid contamination but also, which is of significant importance, to heat denature the whey proteins to get optimal yoghurt structure. This process would be greatly simplified if the pasteurisation of milk could be combined with the pasteurisation of the stabiliser. The stabiliser would then not have to be sterilised separately. Additionally, the method of addition of the stabiliser would be more flexible, since both direct addition as dry mix with sugar and dispersion in a saturated sugar solution could be used as alternatives to the dissolved stabiliser solution.
It is desirable to seek a stabiliser of fermented protein food products that is compatible with proteins in the food material such as milk and which can be added to the food material, resist a pasteurisation together with the food material, prevent flocculation and phase separation during fermentation and finally stabilise the acidified proteins after fermentation and optionally after a final pasteurisation to prolong the shelf-life.
One of the difficulties in providing a stabiliser that may be added prior to pasteurisation, inoculation and fermentation is incompatibility of the stabiliser with the proteins (e.g. milk proteins) at neutral pH. Generally, proteins (e.g. milk proteins) and polysaccharides (e.g. pectin) are incompatible at a sufficiently high bulk concentration and under conditions inhibiting formation of inter-biopolymer complexes. This mainly occurs at a sufficiently high ionic strength (exceeding 0.2), pH values above the protein isoelectric point and at a total biopolymer concentration above 3-4% [1, 12, 16], whereas alkaline pH conditions and low ionic strengths enhance the co-solubility [1, 4]. Furthermore, protein-polysaccharide incompatibility usually increases on heating and with protein denaturation [6, 9, 12, 15]. Therefore, the important pasteurisation of milk, in order to denature the whey proteins before fermentation, would be likely to enhance incompatibility even further in a blend of casein micelles and pectin at neutral pH conditions. The conditions for a limited compatibility are different for systems including neutral (e.g. locust bean gum and guar gum), sulphated (e.g. carrageenan) or carboxylated (e.g. pectin) polysaccharides and the compatibility typically decreases in the order sulphated>neutral>carboxylated polysaccharides [6, 7, 12]. The effect of several hydrocolloids on the stabilisation of casein micelles has been tested with locust bean gum and guar gum of the neutral polysaccharides; gum arabic, CMC (carboxymethylcellulose), pectin, hyaluronic acid and alginates of the carboxylated polysaccharides; and agarose, heparin, chondroitin sulphates, cellulose sulphate, fucoidan and carrageenan of the sulphated polysaccharides. Only carrageenan induced significant stabilisation at pH 6.8 [11].
High molecular weight and rigidity of macromolecule chains tend to increase incompatibility and normally, linear polysaccharides are more incompatible with proteins than branched polysaccharides. In general, the larger the difference in molecular weight and in hydrophilicity, the more pronounced the incompatibility of the biopolymers [12]. The following examples are found in literature:                A system of HE pectin and skimmed milk at natural pH clearly demonstrates depletion flocculation [1, 4, 8]. The destabilisation and subsequent phase separation is even known as a tool to efficiently concentrate proteins from skimmed milk on a technological scale [10]. Depletion flocculation of casein micelles at neutral pH occurs whatever the type of pectin used (low-ester, low-ester amidated and high-ester pectin). The phase separation boundary is obtained at lower polysaccharide concentrations with LE pectin than for HE pectin [16].        Mixing guar gum (neutral polysaccharide) with skimmed milk at neutral pH leads to phase separation, but the phase boundary shifts to higher guar concentrations, when the molecular weight of guar gum is reduced through degradation [17]. Locust bean gum, guar gum and hydrolysed guar gum with reduced molecular weight (all neutral polysaccharides) behave differently in a micellar casein system at neutral pH. Since locust bean gum and hydrolysed guar gum have a lower intrinsic viscosity than the initial guar gum sample, they occupy a smaller volume in the medium per molecule than the guar gum chains. The exclusion of the polymer thus occurs to a lesser extent, resulting in a decreased aggregation of casein micelles at the same polysaccharide concentration [18].        At pH 7, CMC readily precipitates casein from both skimmed milk and from casein model solutions. Less CMC is required when higher viscosity types are used, i.e. types with higher molecular weight [4].        
At present, the only well-known and readily available commercial product on the market for fermented protein beverage applications which can be added prior to fermentation is soluble soybean polysaccharide (SSPS), produced by Fuji Oil [19]. SSPS is a water-soluble polysaccharide extracted and refined from soybean. Fuji Oil Co., Ltd., Japan, has marketed SSPS under the brand name SOYAFIBE-S since 1993. SSPS is mainly composed of the dietary fibre of soybean and has relatively low viscosity and high stability in aqueous solution.
SSPS is a much more branched polymer than HE pectin with a rather short backbone and many more long side chains. HE pectin has a long backbone and just a few short side chains. The component sugars in SSPS are mainly galactose, arabinose, galacturonic acid but also include many others such as rhamnose, fucose, xylose and glucose. Gel filtration chromatographic analysis by HPLC shows that SSPS consist roughly of three components having approximate molecular weights of 550,000; 25,000 and 5,000. The major component of SSPS consists of long-chain rhamnogalacturonan and short-chain homogalacturonan, while citrus pectin consists of short-chain rhamnogalacturonan and long-chain homogalacturonan. For SSPS, homogenous galactosyl and arabinosyl neutral sugar side chains combine with the rhamnogalacturonan region through rhamnose and are longer than the galacturonosyl main backbone.
SSPS has a galacturonic acid content of about 20% [19] whereas pectin has a galacturonic acid content of at least 65%. The anion group of this acid probably binds to the surface of cationic protein particles so that SSPS protects the casein micelles. It is assumed that the adsorbed layer of SSPS is thick, because each molecule is rich in side chains of galactose and arabinose [19]. SSPS is soluble in both cold and hot water without gelation and shows a relatively low viscosity compared to the viscosity of other gums/stabilisers. Acid, heat or salts (e.g. Ca-salts) do not significantly affect the viscosity of SSPS in solution. Under acidic conditions, SSPS prevents protein particles from aggregation and precipitation.
Unlike HE pectin, the point of interest with SSPS is its ability to stabilise protein particles at low pH conditions without raising the viscosity of the acidified protein beverage. SSPS is reported to perform even if applied at an early stage of processing before fermentation, which allows the manufacturing process to be improved. SSPS shows good stabilising effect in lower pH products (below pH4.0). However, SSPS is less effective than HE pectin at higher pH such as around pH4.4 and/or high milk solid non-fat (MSNF) contents.
The need exists to provide alternative stabilisers which may be added during the production of fermented protein products prior to fermentation and preferably prior to the initial pasteurisation.
The present invention alleviates the problems of the prior art.
Statement of Invention
In one aspect the present invention provides a process for the production of a food product comprising the steps of (i) contacting a food material with a stabiliser to provide a food intermediate; and (ii) fermenting the food intermediate; wherein the stabiliser comprises a depolymerised pectin and wherein the food material comprises a protein.
In one aspect, the present invention provides a process for the production of a food product comprising the step of dissolving a stabiliser directly in a food material wherein the stabiliser comprises a depolymerised pectin and wherein the food material comprises a protein.
In another aspect, the present invention provides a food product obtained or obtainable by the process of the present invention.
In a further aspect, the present invention provides use of a stabiliser for improving the texture and/or viscosity of a food product, wherein the stabiliser comprises a depolymerised pectin.
The term “food products” as used herein means a substance that is suitable for human or animal consumption. It will be readily understood that whilst the food product is the product of the process as herein described, it may undergo further processing prior to consumption.
The term “stabiliser” as used herein means a substance which is capable of stabilising protein in a system with which it is contacted—so as to prevent or substantially reduce aggregation and/or sedimentation and/or separation. The “system” may, for example, be a food material comprising a protein, a food intermediate comprising a protein or a food product comprising a protein. Preferably the “system” is a food product comprising a protein.
The term “food material” as used herein means one or more ingredients of the food product.
The term “fermenting” as used herein typically means a process in which desirable chemical changes are brought about in an organic substrate through the action of microbes and/or microbial enzymes. The fermenting conditions typically include attaining and maintaining a specified temperature for a specified period of time. It will be readily appreciated that the temperature and duration may be selected in order to enable the biochemical processes associated with fermentation, especially the breakdown of organic compounds by micro-organisms to progress to a desired extent. The organic compounds may, for example, be carbohydrates, especially sugars such as lactose.
The term “depolymerised pectin” as used herein means a substance obtained or obtainable from naturally-occurring pectin by breaking it down into two or more fragments. Pectin has a backbone comprising repeated structural units and typically has a degree of polymerisation of up to 800 units. These repeated structural units are principally galacturonic acid residues and rhamnopyranosyl residues. The depolymerised pectin has chains of no greater than 250 units, such as chains of 15 to 250 units. Typically these units are galacturonic acid units. The naturally-occurring pectin may be broken down by any suitable depolymerisation method, such as various mechanical, chemical, thermal, enzymatic or irradiative methods or combinations of the same. Suitable depolymerisation methods include those discussed in Studies on Pectin Degradation, W. H. Van Deventer-Schriemer and W. Pilnik, Acta Alimentaria, vol. 16 (2), pp. 143-153 (1987). The term “depolymerised pectin” also includes those substances, for example naturally-occurring substances, which have short chains of 15 to 250 units and in particular short galacturonan chains of 15 to 250 galacturonic acid units.
Advantages
We have surprisingly found that a stabiliser comprising a depolymerised pectin can be applied directly to a protein-containing food material, such as milk, prior to fermentation and yet stabilise the resultant food product which may, for example, be a fermented dairy product.
Prior art stabilisers such as high molecular weight pectin induce phase separation if added to protein-containing food materials such as milk prior to fermentation. Therefore traditionally it has been necessary to add a stabiliser after fermentation in order to achieve the desired stabilisation of the food product.
A further advantage is that the method of addition of the stabiliser is more flexible, since both direct addition as dry mix with sugar and dispersion in a saturated sugar solution may be used as alternatives to the dissolved stabiliser solution.
We have also surprisingly found that a stabiliser comprising a depolymerised pectin dissolves more easily directly in a food material such as milk than other stabilisers such as pectin. The present stabiliser may therefore be dissolved directly in the food material avoiding the need for a separate dissolution step. This further simplifies the production process.
For ease of reference, these and further aspects of the present invention are now discussed under appropriate section headings. However, the teachings under each section are not necessarily limited to each particular section.